Exact time: time zone definition, UTC and GMT, summer time


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Exact time and time difference due to different time zones in the world, definition of UTC and differences with the GMT time, meaning of summer time.


Definition of time zone

When we talk about time zones we want to indicate areas of the world united by the fact of having the same exact time but with differences due to geographic coordinates.

Before they were adopted, local standard time was used which resulted in producing different times for each city or locality on earth.


With the introduction of time zones, this problem was solved, thanks to the setting of the clocks on a solar time calculated by averaging the local solar hours of a given area according to the meridian of the respective time zone in which it falls.

An imaginary representation of this concept can be given starting from the following assumptions:

- Sphericity of the earth.


- Time taken from the ground to complete a complete tour: 24 hours.

- Spherical surface of the earth divided into 24 segments, called time zones, of 15 ° each being the full 360 ° turn of the earth.

- Time taken to travel a segment of 15 ° out of 360 °: 1 hour, the complete rotation being 24 hours.


- The time of each of the 24 time zones is calculated, according to international conventions, referring to the corresponding one of the meridian that passes exactly in the middle of each.

Definition of UTC, GMT time difference

The UTC, initials of English terms Coordinated Universal Time, which translated into Italian means universal coordinated time, is nothing more than the reference time zone according to which the remaining time zones of the earth are calculated.

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Sometimes the term GMT is used, abbreviation of English terms Greenwich Mean Time, in Italian average time of Greenwich, methodology that takes as reference measurements based on celestial phenomena instead of atomic clocks as happens instead for the UTC.

The prevailing current use of UTC is due, in addition to an even greater infinitesimal precision, also to the fact of not having to mention a specific location in a standard that has become international.

As for Italy, CET is used in the solar time period, which is equal to UTC +1, or an hour is added to the base time of the UTC, while when daylight saving time is active you must add 2 hours at UTC, then it becomes UTC +2. Keep in mind that the time zones always coincide with the standard time and do not undergo any change in the summer time period.

Meaning of summer time

To take better advantage of the sunlight during daylight hours, daylight saving time has been introduced, i.e. the convention to keep the clock hands forward for an hour for a period of the year that starts in spring and ends in autumn.

The main purpose of summer time is to save the electricity consumed for public and private lighting, with a consequent better use of all available hours of light which, otherwise, would be partly wasted.

Understanding Time Zones (April 2024)


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